Since the early development of accelerator mass spectrometry in 1977, accelerator mass spectrometers developed into a mature analytic tool for the measurement of extremely low concentrations of long-lived radio-isotopes. Best examples of radio-isotopes that found their application in research fields like archeology, geoscience, environmental science and biomedicine, include 3H, 10Be, 14C, 26Al, 36Cl, 41Ca, 59Ni, 129I and 239Pu.
Historically, the vast majority of accelerators used in accelerator mass spectrometry are tandem accelerators. Although other types of accelerators are sometimes applied, including single-ended accelerators, cyclotrons and linear accelerators, tandem accelerators are the first choice because of their widespread availability, versatility and the high precision that can be achieved with these accelerators.
Known accelerator mass spectrometry systems that use a tandem accelerator comprise an ion source, in which atoms from solid or gaseous sample material are negatively ionized. The negative ions are extracted from the ion source in an electrostatic field region and form a stream of ions, referred to as “ion beam”. The extracted negative ion beam contains the various isotopes of the element of interest. Usually, one of these isotopes is a long-lived radioisotope having an extremely low natural abundance, typically 1010 to 1016 times lower than the stable isotopes. For some elements, including carbon, several stable isotopes are present. Such stable isotope(s) are usually measured in a Faraday cup(s) with intensities of nano-amperes or micro-amperes. However, the concentration of radioisotopes is usually so low that individual ions of these isotopes are commonly counted one after another using a suitable particle detector.
Current systems available on the market include dedicated systems tailored for only one radioisotope (mostly 14C) as well as versatile systems that are capable of analyzing a variety of different radioisotopes with the same instrument. Despite the fact that these systems are carefully designed (they often comprise multiple filter stages, each separating specific interferences from the radioisotope of interest), it is very well possible that unintentionally other particles than the radioisotope of interest make it into the particle detector where they mimic the radioisotopes and increase the background level of the instrument. Since the concentration ratio between a radioisotope and a stable isotope can be as low as 10−16, it is required that current systems perform very well in term of analysis precision and background level reduction.
Reducing the background level of the instrument can be realized by adding one or more filter stages to the system. However, by doing so, costs, complexity and footprint of the accelerator mass spectrometry system are significantly increased.